The present invention relates to an exhaust gas purifying apparatus and method for an internal combustion engine, and more particularly, to an exhaust gas purifying apparatus and method suitable for purifying or removing nitrogen oxides, carbon monoxide and hydrocarbons contained in an exhaust gas discharged from an internal combustion engine which is capable of operating effectively in a lean air to fuel ratio region.
An exhaust gas discharged from an internal combustion engine for a vehicle or the like contains toxic components such as carbon monoxide ( CO ), hydro-carbons ( HC ), and nitrogen oxides ( NO.sub.x ) which are sources of air pollution.
Therefore, extensive research and development on purifying the exhaust gas with catalysts has been carried out. The principle exhaust gas purifying method for the internal combustion engine employs a so-called three way catalyst composed of catalytic substances such as Pt, Rh, Pd and so on to oxidize and purify the HC and CO and to reduce and purify the NO.sub.x.
The exhaust gas purifying method using catalysts is disadvantageous however, in that the exhaust gas is not sufficiently purified until the temperature of the engine rises up to about 300.degree. C. at which the catalyst exhibits its activity, and therefore, the HC mostly produced when the engine starts cannot be substantially purified. Thus, it is desirable to develop a technology for purifying the HC during the period from the time when the engine starts to when the temperature of the engine rises up to the temperature at which the catalyst exhibits its activity.
In order to solve this problem, Japanese Patent Application Laid-Open No. 3-141816 (1991) discloses a method wherein the HC discharged when the engine starts is adsorbed by an adsorbent which is reactivated by its self-heating after completion of the warm-up of the engine.
Incidentally, the three way catalyst can purify the toxic components effectively only in the case where the engine is operating substantially in the vicinity of a theoretical or stoichiometric air to fuel ratio, i.e. the concentration of co-existing O.sub.2 in the exhaust gas is approximately 0.5 vol % or less. As well known, a narrow region in the vicinity of the stoichiometric air to fuel ratio including this is called a window. In view of this, although the air to fuel ratio for a vehicle changes according to the operating condition, the upper limit of the air-to-fuel A/F has been set at approximately 14.7 which is a stoichiometric air to fuel ratio, where A represents the total weight of air and fuel and F represents weight of fuel.
However, if the engine may be operated in an air to fuel ratio region leaner than the theoretical air to fuel ratio, i.e. lean region, fuel cost can be decreased and the amount of toxic components in the exhaust gas can also be decreased. For this reason, a "lean burn" combustion technology has been developed. In recent years, there is a strong tendency for the range of air-to-fuel ratio to widen toward a leaner region (22 .about. 24 or more measured by A/F ).
Accordingly, a NO.sub.x purifying or removing technology suitable for the lean burn combustion, (i.e. a method to remove NO.sub.x in the presence of oxygen) is widely studied, and a method is thought to be promising, has been developed in which NO.sub.x is reduced by using hydro-carbons as a reductant with a catalyst. For a catalyst for removing NO.sub.x using hydrocarbons as a reductant in the presence of oxygen, i.e. a lean NO.sub.x catalyst, zeolite carrying a transition metal such as copper is described, for example, in Japanese Patent Application Laid-Open No. 1-130735 (1989), and systems utilizing the catalyst of this kind is described in Japanese Patent Application Laid-Open No. 4-175416 (1992) and Japanese Patent Application Laid-Open No. 3-225013 (1991).
The method of using the lean NO.sub.x catalyst and the three way catalyst when the air to fuel ratio is in the lean or rich region as described in Japanese Patent Application Laid-Open No. 4-175416 (1992) or Japanese Patent Application Laid-Open No. 3-225013 (1991). However, the purification of the HC discharged when the engine starts is not suggested.